The present invention relates to a semiconductor apparatus which includes a conducting path formed from fine particles that include a conductor or a semiconductor and organic semiconductor molecules, and a manufacturing method for fabricating the same.
A thin film transistor (hereinafter, referred to simply as “TFT”) is widely used as a switching element in an electronic circuit, especially in an active matrix circuit for use in display and the like.
Currently, most of TFTs are Si inorganic semiconductor transistors using amorphous silicon (a-Si) or polycrystalline silicon (Poly-Si) in a semiconductor layer (channel layer). In the fabrication of these transistors, plasma chemical vapor deposition (hereinafter, referred to simply as “CVD”) or the like is used in formation of a semiconductor layer, and therefore the process cost is high. In addition, a heat treatment at a temperature as high as about 350° C. is required, and hence, not only the process cost is increased, but also the substrate is restricted.
In recent years, the development of organic semiconductor transistors using an organic semiconductor material is being vigorously made since such transistors can be fabricated by a low cost process and film deposition on a flexible substrate with low heat resistance, such as a plastic, is possible.
Organic semiconductor materials advantageously enable preparation of TFT at a lower temperature at a lower cost, e.g., by spin coating or dipping. However, they can achieve only a typical mobility value of 10−3 to 1 cm2/Vs wherein the mobility is an index of the TFT properties. See, C. D. Dimitrakopoulos et al., Adv. Mater. (2002), 14, 99. This value is small, as compared to several cm2/Vs which is the mobility of α-Si, or about 100 cm2/Vs which is the mobility of Poly-Si, and does not reach a mobility of 1 to 3 cm2/Vs required in the TFT for display. Therefore, the improvement of mobility is an important task of the development of organic semiconductor materials.
The mobility of the organic semiconductor material is determined by intramolecular charge transfer and intermolecular charge transfer.
The intramolecular charge transfer can be achieved by a conjugated system formed from delocalized electrons. The intermolecular charge transfer is made by conduction by overlapping of molecule orbital due to bonding between molecules, i.e., van der Waals force, or hopping conduction through an intermolecular trap level.
In this case, when the intramolecular mobility is taken as μ-intra, the mobility by bonding between molecules is taken as μ-inter, and the mobility of intermolecular hopping conduction is taken as μ-hop, the relationship: μ-intra >>μ-inter>μ-hop is satisfied. In the organic semiconductor material, slow intermolecular charge transfer limits the collective mobility, thus lowering the charge mobility.
For improving the mobility of the organic semiconductor, various studies have been made.
For example, in the formation of a pentacene thin film comprised of an organic semiconductor material by a vacuum vapor deposition process, the deposition rate in vapor deposition is considerably suppressed and the substrate temperature is lowered to room temperature to improve the orientation of the molecules, thus achieving a mobility of 0.6 cm2/Vs. See, for example, C. D. Dimitrakopoulos et al., IBM J. Res. & Dev. (2001), 45, 11.
This improves the material in crystalline properties to suppress the intermolecular hopping conduction, improving the mobility. The mobility is improved, but the intermolecular transfer limits the collective mobility as well, and a satisfactorily large mobility cannot be obtained.
As an organic semiconductor transistor positively utilizing the intramolecular charge transfer, Lucent Technologies has proposed a self-assembled monolayer field-effect transistor (SAMFET). In this method, a semiconductor layer comprised of a self-assembled monomolecular film is formed between a source electrode and a drain electrode to realize an FET having a gate length of 1.5 nm.
In this method, the channel is formed from a monomolecular layer oriented in the direction of linking the source electrode and the drain electrode, and therefore charge transfer in the channel includes only intramolecular transfer, thus achieving a mobility of 290 cm2/Vs, which is larger than that of Poly-Si. See, for example, J. H. Schoen et al., Nature (2001), 413, 713; and Appl. Phys. Lett. (2002), 80, 847.
However, in this channel structure, the gate length is determined by the thickness of the monomolecular film, and hence the gate length is as very small as several nm and therefore the source-drain withstand voltage is lowered, causing a problem that a high driving voltage cannot be obtained. In addition, for preventing the monomolecular film from being broken, it is necessary to cool the substrate to a temperature of −172 to −30° C. during the formation of electrodes on the monomolecular film, and therefore the process cost is increased, and this method is not practical.
Further, International Business Machines (IBM) has proposed a channel material using an organic/inorganic mixed material. See, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 2000-260999. In this method, the inorganic component and the organic component form a layered structure, and, while utilizing the high carrier mobility properties of the inorganic crystalline solid, the action of the organic component to promote self-assembly of the inorganic material is utilized to enable the material to be deposited on a substrate under low temperature conditions for the treatment.
A mobility of 1 to 100 cm2/Vs is expected, but the mobility actually achieved is only 0.25 cm2/Vs. This is a mobility higher than that of the organic semiconductor generally formed by spin coating, but it is equivalent to the mobility of the organic semiconductor formed by vapor deposition or the like, and a mobility higher than that of a-Si has not yet been obtained.